Project Abstract Obesity produces adverse health consequences such as dyslipidemia, cardiovascular disease, insulin resistance, and Type 2 diabetes. Obesity has become a heavy social burden as about 500 million adults worldwide are now considered obese. Leptin is the critical adipokine that maintains energy homeostasis and body weight by modulating feeding behavior and energy expenditure. In most cases, plasma leptin levels are abnormally higher in obesity patients than in normal individuals (leptin resistance), thus administration of additional leptin fails to reverse the obese state. It is essential to understand the molecular mechanism and regulation of leptin resistance for the effective leptin therapies. During a search for factors that affect leptin sensitivity and body weight, we identified Slug (also called Snai2) epigenetic factor. Slug elicits deacetylation, demethylation, and/or methylation of H3K4, H3K9, and/or H3K27, thereby repressing its target genes. However, its action in the brain has not been explored. The preliminary data indicate that Slug-expressing neurons are highly enriched in a subset of hypothalamic neurons which are implicated in regulating energy balance and body weight. A high fat diet (HFD) increases both the levels of hypothalamic Slug and the number of hypothalamic Slug+ neurons. Importantly, both global (KO) and LepR+ cell-specific Slug knockout (LKO) mice resist HFD-induced leptin resistance, obesity, type 2 diabetes, and nonalcoholic fatty liver disease, owing to increasing energy expenditure. My working hypothesis is that hypothalamic Slug+ neurons, particularly the Slug+LepR+ subpopulations are the hub of the energy metabolism circuits. At the molecular level, Slug epigenetically regulates expression of key molecules involved in leptin signaling. To test this hypothesis, I have developed two aims. Aim 1 is to delineate the anatomic, chemical, and functional properties of Slug+ neurons. To determine hypothalamic Slug+ neural circuits, I will map the upstream and the downstream of Slug+ neurons using Cre/loxp-dependent and viral-based neural tracing techniques. In addition, I will identify signature neuropeptides expressed by Slug+ neurons in order to gain insight into the mechanism by which the Slug+ circuitry controls energy metabolism and body weight. Furthermore, I will define the distinct function of Slug+ neurons using chemogenetic approaches. Aim 2 is to interrogate the molecular mechanisms by which Slug controls the ability of the Slug+LepR+ circuits to regulate energy balance and body weight. My preliminary data suggests that Slug likely inhibits leptin signaling, leading to leptin resistance. To extend these exciting findings, I will assess hypothalamic leptin signaling in LKO mice. I will test the hypothesis that Slug epigenetically suppresses expression of leptin receptor, contributing to leptin resistance. Additionally, I will analyze the translational profile of hypothalamic Slug+ neurons in order to comprehensively understand Slug+ neuronal behavior. The impact includes defining a novel Slug circuitry and unveiling novel epigenetic regulation of leptin resistance. The outcomes of this project are expected to lead to new therapeutic strategies for obesity prevention/treatment by targeting hypothalamic Slug.